Central Pain Modulation

The Dorsal Horn

Sensory neurones enter the spinal cord via the dorsal spinal roots and synapse with cells in the gray matter of the dorsal horn. The gray matter is divided into ten layers, or laminae, some of which are involved in pain transmis sion. In particular, polymodal C-fibers enter into laminae I, II and V, with A-fibers entering into laminae I, II, V and X. The laminae have direct inputs from the periphery and also connections with other laminae; thus, information is not only received in the dorsal horns, but is also subject to a degree of modulation before onward transmission. Neurotransmitters within the dorsal horn include the excitatory amino acids glutamate and aspartate, acting via N-methyl-d-aspartate (NMDA) channels, substance P and calcitonin gene-related peptide (CGRP). These substances not only stimulate second-order neurones, but also increase the response of cells to noxious peripheral stimuli.

Ascending Pathways

Information is conveyed to the brain by the primary nociceptive pathways, consisting of the spinomesencephalic tract, the spinothalamic tract and the spinoreticular tract.

The lateral system consists of the lateral spinothalamic tract. It consists of long, myeli-nated axons that are rapidly conducting. There is a discrete somatic organization within these pathways, allowing information on the precise location of the stimulus, as well as intensity and duration of injury. The pathway relays in the thalamic nuclei before projecting onto the sensory cortex.

The medial system consists of the medial spinothalamic tract, the spinoreticular tract and the spinomesencephalic tract. These tracts contain both long and short fibers, are multisy-naptic and relatively slow to conduct. The system lacks organization and is thought to be responsible for the diffuse, persistent quality of pain. The system relays in the reticular formation, peri-aqueductal gray matter, hypothalamus, medial and intra-laminar thalamic nuclei,

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